Fix broken dependency-mongering for index operator classes/families.

For a long time, opclasscmds.c explained that "we do not create a
dependency link to the AM [for an opclass or opfamily], because we don't
currently support DROP ACCESS METHOD".  Commit 473b9328 invented
DROP ACCESS METHOD, but it batted only 1 for 2 on adding the dependency
links, and 0 for 2 on updating the comments about the topic.

In passing, undo the same commit's entirely inappropriate decision to
blow away an existing index as a side-effect of create_am.sql.